A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The demand for ever-smaller features with smaller pitches to be imaged with lithographic apparatus such as steppers and scanners has resulted in the use of projection systems with increasing numerical aperture (NA). The angle of rays of radiation within the projection apparatus with respect to the optical axis increases with increasing NA and decreasing feature pitch. The vector nature of radiation becomes important for imaging these features because only identically polarized components of electromagnetic waves interfere. Therefore, it is not the wavefront quality alone that determines the image contrast; also the polarization of the radiation beam has a considerable influence on image contrast.
Conventionally, a radiation source used for a lithographic apparatus outputs a linearly polarized radiation beam. In conventional lithographic apparatus, polarization changing elements have been implemented to change the polarization state of either an entire cross-section or segments of the cross-sections of the radiation beam. Such polarization state changing optical components may use optical retarders, for example quarter lambda or half lambda plates.
US 2006/0170901 A1 describes a lithographic apparatus in which a polarization direction of segments of the radiation beam are adjusted using a polarization-modulating element. The polarization-modulating element contains segments of optical active material of different thickness for inducing a fixed adjustment of the polarization direction for each individual segment of the radiation beam.